Tuned electric circuits



Jan. 9, 1940; I E. s. BEALE rfmmn ELECTRIC CIRCUITS Filed July 1, 1938 2 Sheets-Shet 1 v Fig- I. T LOW PASS FILTER AMPLIFYING DEVICE HIGH PASS FILTER l/fenuaf/on Frequency A Af/enual'fon Frequency Arman-"Y Jan. 9, 1940. E. 51.. BEALE 2,136,571

' TUNED ELEp Rm CIRCUITS v Filed July l, 1938 2 Shets-Sheet 2 Fig. 2.

\ uws/vrm fiTTOR/VEY Patentedjan. a a f v Q j 2,186,511" f I a v TUNED sclicrnio'clnciursf I Evelyn stcwart 'i llansdowne Beale, chelseaa London, England, \as'si'gnor tolnternational standard Electric Corporation; New York; N.v Y. Application July 1, 1938, Serial 15101216319 l j a In Great Britain July 27, '1937 5 a v I a a 1112o1aims.(ci.25o s6) I s ::'I'hislinvention relates. to an electriccircu-it reployed this specification refers to ir'equencies sponsive'to low frequencies and which may-be f theo der 0f 00 ycl l'pfi Second an f fi wadapted to operate either as a frequency selec-f 1 Although the iundangental circuit described tiveznircuit ior low frequencies or as a circuit f 'r g th appl a i the e as d s ribed apable of generating I oscillations of low frecontains a ingh'passfilterin the feed back cir- 5 1 quency. v s I cuit and a'low-pass filter in the amplifier circuit It ;is well known, that in" circuits for passing a Whichtonsists (if a ingl section mad low frequencies difficulties are usually encoun- 19.: Of a'resi ance a a ap c y, the invention teredwover'the design :of the inductances'which' a s n l s 0W equencyi p deViCQ I i ,10 tendtolbecome heavy and bulky-if they are to co prising a fundamental circu t i which high 10 i I have a suificiently-lhigh ratio of reactance to repa s te s n t e amplifier circuit and the low Sistance to be use ful'; It'E'istherefQre an object p fi te in the k Circuit [With th 0f the .present invention to overcome the above latter r a e there would b a q y of minimum attendntioni'gasdescribed. The ini disadvantages. :x

.15 The, underlying idea of'ithe p sentvi'nventi n vention will hereinaiterbe described 'with refer- 1 5 is to provide a low frequency responsive device 61106300 t e a e t h ow in t e drawings, including what maygbe' called a fundamental c i1' in wh h W P lt E in t e pl fi Quit capableof operating in a manner similar to circuit, but it'must be app ated thatithe-ina-tuned circuit and responsive to low frequencies, 've tion embo es 18 5 t a t ve arrang- $20 the said fundamental circuit containing resistment; j a v ances and capacities but no inductances. v 1 Order that i en y e e fly' gi ordin t a, feature of the presentinvenunderstood reference directed to the iaccom vtier; thereiis provided an amplifier comprising a pany n sw ch showl d a amm a y fundamentalicircuit containing no inductances and by way i p bfl e s 0f t eprese but resistances-and capacities so arranged as to ent invention 5 91m ailow passasection and a high pass section, ln'thedrawings: v I g v a the, low pass section being in the amplifier 'cir- T Fig-VlV/ShOWS i rammatically one' foim 0f quit and the, high pass. se ti 'b {fe d what is .referred.toi;herein as a-.fundamenta1' cirback cireuit, .thenet gain of itheiamplifier beingj "wi 7 n v I a adjustedso that the minimumofithevresulfiant rFig.;{2 shows a simplevform of circuit embody-,- 3,0 ttenuation .curvenof the two sections-pf the in thelfundamentaicircuit shown nE 1; m g mm it s- 1 t u tt t ..Fig.-3"'shows amorei practical arrangement of up to the level of zero attenuation, wherebyuthe c u t than h ow-r1 84 mo e p r i fiundamentalcircuit operates in a manner anallarly fOJI-HSEJELS a..tuned." amplifier; 'ogousto a tuned circuit responsive to the Yreso-= igSIB I, 311,38 p ss flw the attenuae. 35,

nant frequency. H 1 k V v tion curves for the fundamental circuit. 1'

T e invention also includes an oscillator for e g briefly 'toxFig. L Will be Observed enerating low frequency oscillations, characterthat th m t l circuit'comprifies' an input ised in this, that the oscillator comprises a fun- {Bran u pu lla 10W 'pa L ahighv 7 40 damental circuit containing ,no inductances' but". pass sfiltel" ER i 'n mp y dev 40' resistances and capacities so arranged as to form The c g -pass circuit wh l l s a feed-back ahigh pass section forming a feed back cirfil omp isesa cond nser-Granda re s n e cult and a l w passsection forming part Ofth R2 r(see, Fig,."and thelow pass filter which amplifier circuit, the net gain of the amplifier fihnartof thezamp yi e fi ts p fl bein increased until the attenuation of th fun-v, awn-gens t resistance 31 (S59 45 damental circuit is substantially r ywhereby The attenuation/curries oi rthe'two sections of the circuit is rendered unstable and: starts to thefimdamema'l circuiifmgether W b oscillate a the fr qu n y of minimum" t or, resultantlcurves are showninFigshBa, 3b,3jc. n r Y a; Infli'ia, 3a,. the curve. markedn issthe attenua- It will be understood; therefore,'from the above 3 u C131 m 50 I that the-expression flow frequency" responsive attenufalhon-curvei 3 Whilst the 4011mm device in this specification includes an amplifier C m h "t a n iv selective of low frequencies and also anoscillator 0 t mi is-x Ifthe 1 v 7 for generating low frequencies. It may'also-be I {a 0 -12 1 v "mentioned thatthe term-low frequency "as em- Y f 1 q frequency be chosen correctly, that is to say, in the region of unity (1) the resultant curve has a rounded form with a definite minimum (see Fig. 3a) On the other hand if the ratio is too high (Fig. 31)) or too w (Fig. 3c)- the come a time when theminimuin of the curve reaches or approaches very closely to the level of zero attenuation. At this point the circuit becomes unstable and will start oscillating at the frequency of minimum attenuation of the curve indicated at Dr The property is utilised in a low frequency responsivedevice in the] form of an oscillator in accordance with the present invention for generating. oscillations of low frequency, for example, for test purposes and it 1 may be mentioned that for all practical purposes there is no particular lower limit to the frequency that may be obtained owing to the absence of inductances in the circuit. The frequency of the oscillations produced is given by the following formula, where j is the resonant Therefore by a suitable variation in the values of the resistancesand/or the capacities any desired testing frequency may be produced. Frequencies as low as 0.1 cycle per second can be produced with quite convenient values of resistanceand capacity.

Considering now the application of the fundamental circuit to j a low frequency responsive device in the-form of an amplifier for. the selection of a particular frequency letit be supposed that the gain of the amplifier is critically adjusted'lin such a manner that the minimum of the resultant attenuation curve, i. .e. thecurve C comes close to, but not quite down to; the level of zero attenuation. If this particular frequency be fed into. the amplifier it will be amplified to a considerably greater. extentthan any otherfrequency; The increase in attenuation due to the high pass and low pass circuits alone on each side of the resonant frequency is onlyfiabout 2 decibels atone half and at twice the resonant frequency, but this discrimination can theoretically be increased indefinitely by increasing the gain of the amplifier and so reducing the minimum attenuation to an infinitesimally small value as described without overstepping' this point .into the region of self oscillation. There are, of course, certainpractical limits to this increase in discrimination governed chiefly by the stability of the gainiin the amplifier. It seems from theoretical calculations that this discrimination could be increased to 30 or 40 decibels without great diifioulty. The resonant frequenoyin this application is of course given by the same formula as for the production of oscillations mentionedabove. 1

Turning nowto consider the circuits in rather more detail Fig. 2 shows a simple way in which the fundamental circuit may be embodied in an amplifier but while thisjcircuit arrangement is matically in Fig. 1.

convenient from the point of view of explanation of the invention it is not an entirely convenient embodiment as will be explained in a moment. In Fig. 2 the source of voltage is connected be tween the input terminals I and II and the ter-' minal I is connected directly to the grid eleo tr odeof a triode valve V1. The anode of this valve V1 is connected through a'resistance R6 to the positive pole of a suitable source of potential and through a condenser C4 to the input of the fundamental circuit of the kind shown diagram- The input of the funda mental circuit is connected through a resistance R7 and a source of grid bias GB2 to earth and also through the resistance R1 of the fundamental circuit to the grid electrode of a second triode valve V2. Connected between the resistance R1 and the grid of V2 is a lead to the condenser C1 of the fundamental circuit which is in turn connected to earth, the resistance R1 and the condenser C1 forming a low pass section of the fundamental circuit.

The cathodeof V2 is connected to the cathode of V1 andalso to earth in the usual manner. The anode of V2 is connected toa source of potential through the resistance Re and through a condenserCs to the output 0. .The anode of V2 is also connected through the condenser C2 and the resistances R2, R3 to earth; The junction of the resistances R2. R3 is connected to the input of the valve V1. In this way a part of the output potential developed across the resistances R2 and R3 namely that developed across the resistance R3 is fed to the grid of V1. The amount fed back can be adjusted by varying the value R3 and for fications required to make the above circuit suit-- able for practical use andFig. 3 shows the samecircuit embodying two modifications which randerthearrangement a practicalone. The objection to the Fig. 2 circuit is that neither of the input terminals I or II can be connected to earth since-this would short circuit the feed-back circuit reistance R3. It will be understood that there are a number of ways in which Fig. 2 may be modified and that Fig. 3 shows one convenient way. 1

In Fig. 3 the feed back resistance R3 is included as a cathode resistance for V1 thus making it possible to apply the inputbetween the grid of V1 and earth; The thirdstage of amplification has been included primarily in order to obtain a phase reversal since feed-back in the opposite phase is required when the resistance R3 is put in this position. This would be possible with either one stage or three stages butthere areseveral advantages in using three stages. Firstly, the combination R2 R3 acts as a potential divider to obtain the correct amount of feed-back and if there is plenty of gain in the amplifier within the fe'ed v disadvantage and in fact, might be useful, but if negative feedback is to be used in this stage it would be-better to includethis outside the positive, feedback path, i-.' e. in;the form of. another sasis :w'ell known,.rone of the. zch'aracteristics =of dimer-reason for making R3 small .isthat a change value :notzthen change the i valueLCzRa substantially. The:.-extra stageuofvamplification may very welli berre'quire'd in '1 any case touobtain, adequate output from a smallisignal sothat from this'pointotaview'itis useful. i

:The additionalgain.iprovided in this way has :another Verynefinite advantage in that it :makes .:possible the :use :of a considerable degree of negative feed-back. in the amplifier. This wouldgappeartobeparticularly1useful inlthis case because negative feedback is thatvif a. large amountgof negative feedback is used the net gain of the amplifier depends only on the properties of thefeed-back circuit and not on the stage gain of the amplifier. It should therefore-bepossible to make the efiective gain' nearly independent of changes "of valve ivoltag'es and so .on, thereby obtaining stable operation: oi the positivefeedback. In the "Figure 3 circuit the negative feedback is applied via Brand-R5 around the last two-stages only. There-are many other ways in which it .couldbe #employed but the negative feedback must i obviously not include either section of the funda- -Tnenta1 circuit R1 -C1*and-Rz ('12. The frequency of the-oscillator or tuned :t'amplifier -selective--of a particular frequency is -very simply varied'iby altering R1 and R2 or C1 land-"0.2 *inisuch a way-as to maintain the ratio substantially constant and equal approximately to unity (1.0). One very convenient Way of effecting this in such a Way as to cover a wide frequency range is to make R1 and R2 continuous.-

1y variable in the form of apair of gangedresistances and Grand: C2 variable in steps witha pair of ganged studswitches. As an example of such an amplifier tuned to 50 cycles per second An amplifier of this type could also be given band pass properties by making the ratio quency in the same way that ordinary tuned cir-.

cuits are arranged either to pass a single frequency or a band of frequencies.

The selectivity of the fundamental circuit could be improved by employing more than one sectionin one. or eachfiltenfor example,- the low pass filter mightbe composed of. two L sections, each consisting of a series resistance and a, shunt capacity so as to give a double filter with agreater degree of attenuation above thecut-off fre quency than a single section. Similarly, the high pass filter might be composed of two sections each consisting of a series capacity and a shunt resistance. Such an arrangement would lead to greater phase changes in both filters than with a, single section for each and would therefore tend to turn the positivefe'ed-back into a negative harmonic content in the l'output 'Ifor bobvious plifying' device to the input circuit thereof,- and a of the'two sections of the fundamental circuit comes close to but not quitedown to the. level of circuit responsive to the resonant frequency.

,ities so arranged as to form a high pass section feed-shack atsa :frequency;siearitoyrthefrequency of minimumnattenuation',rand itiwvould-iappear that this fact combinedwith the greater curvature of :the attenuation curves should give distinctly greater selectivity of the device as a whole.

-Onel rather obvious point which-should be -b'orne'inmind when put-ting this'device into prac-' tical operation is that it is desirable to-arrange the operating conditions for the amplifying valves "in such a way "that they are working on apart :of p10 their characteristic which gives 'the maximum value for-"the amplification. The reason' for this that the-tgain of the amplifier would then not increase with theamplitude of the input orout put' signal. If this'were to occur and the gain of .15 theemplifienhad been adjusted so as to be just on the -ipoint-"of self oscillation, a large input signal would increase the gain-of the amplifier and 'this-woul'd result in the system actuallysgoing'intoastatefof oscillation. Corresponding-ly go' :when the circuit is used as an oscillator it would usua'llyibeudesirable for the amplituderof-sthe oscillation to increase gradually as the feedback "is' increased beyond the-critical value. 'I hiswould occur if mat-ters were arranged as describedm2 "whereas the amplitude ofithe oscillation would "suddenly rise' f-rom zeroi'to adarge value; and. in "all pre'babilitythere vvould' 'be a considerable reasons.

Whatis claimed is: I l 1. A'low trequency responsive device comprising an amplifying-device having an amplifier :circuit, a feedback path from the output of said am- 35 fundamental circuit for said amplifying device containing only resistance and capacityelements so arranged as toform a high pass section and a low pass section, one of said sections being in the feedback path,'and the other of saidsections being in said amplifier circuit. v

2. A low frequency amplifier, characterised in this, that the said amplifier comprises a fundamental circuit containing no inductances but re-' sistances and capacities so arranged as to'form' a low pass section and a high pass section, the low pass section being in the amplifier circuitand-the high pass section being in a feed back circuit, the I net gain of the amplifier being adjusted so that the minimum of the resultant attenuation curve zero attenuation, whereby the fundamental circuit operates in. a manneranalogous to a tuned- 3. An oscillator'for generating low frequency 55 oscillations, characterised in this, that the oscillator comprises a fundamental circuit containing no inductances, butresistances and capacforming a feed back circuit and a low pass section forming part of the amplifier circuit, the net gain of the amplifier being increased until the "attenuationof the fundamentalcircuit is substantially zero whereby. the circuit is rendered unstable and starts to oscillate at the frequency of minimum attenuation.

'4. A low frequency responsive device according to claim 1, wherein the low pass'section is in the feed back circuit and the high pass section is section of the fundamental circuit contains a capacity 02 and a resistance R2, the ratio being chosen in the region of unity (1) so that the resultant attenuation curve has a rounded form with a definite maximum. r

6. An amplifier according to claim 2, characterised in this, that the discrimination of the fundamental circuit on either side of the resonant frequency is increased to a vvalue above that given by the high pass and low pass sections of the fundamental circuit alone.

7. A low frequency responsive device, according to claim 1 in which said amplifying device includes a plurality of vacuum valve stages, characterised in this, that the feed back path comprises a resistance (R3) is included in the cathode circuit of the first Valve for the purpose specified, an odd number of stages of amplification being provided in order to obtain the correct phase for the feed back.

8. A low frequency responsive device according to claim 1 in which said amplifying device comprises a plurality of stages further comprising a negative feedback circuit for said amplify ing device to obtain particularly stable characteristics in the amplifier, arranged around the last two stages of amplification.

9. A low frequency responsive device according to claim 1, characterized in this, that the resistances of the fundamental circuit arecontinuously variable, and the capacities are variable whereby said fundamental circuit may be adjusted'so said device may operate as a low frequency amplifier responsive to any particular frequency or a low frequency oscillation responsistanceRz and is included in said feedback circuit, the values of the capacities and resistances being such that the ratio q is appreciably different from unity, whereby said device has a band pass frequency characteristic.

11. A low frequency responsive device according to claim 1, wherein said low pass section comprisestwo portions each including a series resistance and a shunt connected condenser, and said high pass section comprises two portions each including a series condenser and a'shunt resistance.

12. A low frequency responsive device according to claim 1, characterized in that the resistances of said fundamental circuit are made in the form of variable gauged resistances, and said capacities are made variable in steps by means of gauged stud switches.

EVELYN STEWART LANSDOWNE BEALE. 

